A two stage cascaded converter converts a digital word into a corresponding analog voltage. The first stage includes a series string, or ladder of resistors coupled across two supply voltages for dividing the voltage into a number of incremental voltages. The first resistor string resolves higher order bits of the digital input or control word and the second resistor string resolves lower order bits. The first stage resistor string is coupled to the second stage, typically also a resistor string, by a plurality of switches and buffer amplifiers. Such arrangements are shown in U.S. Pat. Nos. 4,543,560 and 3,997,892, incorporated herein by reference. Generally, buffer amplifiers have offset voltages associated therewith and require trimming in order to obtain satisfactory conversion accuracy.
U.S. Pat. No. 3,997,892 to Susset, shows a two-stage cascaded converter wherein the first stage includes a resistor-string to produce a first voltage corresponding to a set of higher-order input bits. The second converter stage is another resistor-string arranged to produce a second voltage corresponding to a set of lower-order bits. The voltage across a selected resistor of the first stage is applied to the ends of the resistor string of the second stage, so that the latter stage produces an output effectively interpolating the selected first-stage segment voltage in accordance with the lower-order bits.
Converters such as shown by Susset have an important advantage in that they are inherently capable of monotonic performance. However, the Susset converter is practical only for use in relatively low-resolution applications. This is because the selector switch system used to make connection to the resistor string would become prohibitively large and complex for a high-resolution monolithic converter such as one capable of resolving a 14-16-bit input word. For example, the first stage of such a converter typically would have a 256-R resistor string. The complexity of the switch selector system for such a resistor string especially results from the fact that it must be able to select any pair of adjacent voltage taps of the string to produce the segment voltage for the second stage converter.
In U.S. Pat. No. 4,543,560 to Holloway, a new selector switch tree structure was provided for accessing the voltage tap points of a resistor string having 256 resistors. This tree structure made it possible to use common control lines for most of the two sets of switches on opposite sides of the resistor string, and leading respectively to alternate voltage taps of the resistor string. The selection of adjacent pairs of taps is thereby effected with a minimum of additional circuitry required to control the two sets of switches. However, this improvement to Susset requires that the voltage of every resistor tap pass through four series-connect switches. It also required the use of operational amplifiers (Op-Amps) which introduces problems with linearity. The use of op-amps is particularly troublesome when the voltage outputs of the first stage have alternating polarity that is then applied to the second resistor string.